The invention relates to anionic polymerization resulting in very high molecular weight, highly branched diene polymers. More particularly, the invention relates to novel multiply-metalated, solid supported anionic polymerization initiators that are useful for producing such polymers. The macro-branched polymers synthesized by the process of the invention exhibit desirable properties, such as the ability to absorb hydrocarbon solvents and oils, and are readily compounded to form vulcanizable elastomeric compounds and articles that have excellent resistance to wear and tear and exhibit reduced hysteresis properties.
When producing polymers for use in rubber articles, such as tires, power belts, and the like, it is desirable that these polymers are easily processible during compounding and have a high molecular weight with a controlled molecular weight distribution, glass transition temperature (Tg) and vinyl content. It is also desirable that reinforcing fillers, such as carbon black, be well dispersed throughout the rubber in order to improve various physical properties. This dispersion can be achieved, for example, by end capping polydienes by reacting a metal terminated polydiene with an end capping agent, or by utilizing functionalized anionic polymerization initiators such as lithium-based amine or amide initiators that incorporate a functional group onto one or both ends of the polymer chain. Rubber articles produced from vulcanized elastomers exhibiting these properties, will have reduced hysteresis resulting in an increase in rebound, a decrease in rolling resistance and less heat build-up when mechanical stresses are applied.
Anionic polymerization initiators based on lithium are well known for producing linear polydiene homopolymers and copolymers. Lithium-based macrocyclic anionic polymerization initiators have also been described in U.S. Pat. Nos. 5,677,399 and 5,700,888. These initiators are utilized to form stable macrocyclic polymers that have low viscosities at high molecular weights and thus provide for enhanced polymer processibility during molding, extruding and the forming of films. Such polymers can be compounded to form vulcanizable elastomeric compounds and articles that exhibit reduced hysteresis properties.
There is still a need, however, for anionic polymerization initiators that can be used to synthesize high molecular weight linear polymers that are easily processible and have the desirable properties described above.
The present invention provides novel solid supported anionic polymerization initiators that are multiply metalated and are useful for producing extremely high molecular weight branched diene polymers. In particular, an advantage of the invention is that the macro-branched diene polymers are prepared using the invention initiators under normal conditions for an anionic polymerization process while the initiators are in suspension in a solution of monomers. Moreover, the initiators of the present invention are also suitable for use in gas phase anionic polymerization of conjugated diene monomers, as disclosed in U.S. Pat. No. 6,071,847, the disclosure of which is hereby incorporated by reference. Because of their extremely high molecular weight and controlled molecular weight distribution, Tg and vinyl content, the polymers produced by anionic polymerization employing the invention initiators are useful for producing many different high performance vulcanates. The macro-branched polymers synthesized by the process of the invention also exhibit other desirable properties, such as the ability to readily absorb hydrocarbon solvents and oils, and they are easily compounded to form vulcanizable elastomeric compounds and articles that have excellent resistance to wear and tear and exhibit reduced hysteresis properties.
In particular, the anionic polymerization initiators of the invention have the formula P(Me)n, wherein P is a metalatable particle having a diameter of about 1 micron to about 1000 microns comprising a thermoplastic polymer or a cured elastomer. The particle is multiply-metalated with a Group IA alkali metal (Me). The number of metal atoms covalently bound to the particle ranges from n=3 to n=a multiplicity of atoms, 10X (e.g., 1010). The alkali metal atoms bonded to a single particle may all be the same or may be different from each other. The metal atoms may be any Group IA alkali metal including lithium, sodium, potassium, rubidium, cesium and francium. Preferably the metal atoms are selected from lithium, sodium and potassium and, more preferably, are a mixture of lithium atoms and at least one of sodium atoms and potassium atoms. Most preferably, all of the alkali metal atoms are the same and are lithium atoms.
As used in the context of the invention, the term xe2x80x9cmetalatedxe2x80x9d refers to an acid:base reaction, known to those skilled in the art, involving the transfer of a metal atom from a strong base to a more acidic polymer with the concomitant transfer of a hydrogen atom from the polymer to the base, thus forming a polymer carbon-metal covalent bond. A xe2x80x9cmetalatablexe2x80x9d thermoplastic polymer or cured elastomer is one that can participate in this reaction and become metalated.
The above described anionic polymerization initiators are used to homopolymerize conjugated diolefin monomers having from about 4 to about 12 carbon atoms, and to copolymerize the conjugated diolefin monomers with monovinyl aromatic monomers having from about 8 to about 20 carbon atoms, to prepare a macro-branched diene homopolymer or copolymer having the formula
P-[(polymer)-Me]n 
prior to quenching, wherein P, Me and n are the same as previously described and (polymer) represents a polymer chain covalently bonded to the particle. The polymerization reaction is terminated with a terminating or a functionalizing agent. The resulting polymers have a low Tg, preferably less than xe2x88x9220xc2x0 C., more preferably less than xe2x88x9230xc2x0 C., and most preferably less than xe2x88x9235xc2x0 C., and are readily usable in rubber products such as tires.
The invention provides the initiators, the macro-branched diene polymer, a vulcanizable elastomer composition formed from the polymer, and a tire having at least one component formed from the vulcanizable elastomer composition. The vulcanizable elastomer composition may comprise the invention polymer only or may comprise a blend of the invention polymer and at least one other polymer, such as styrene-butadiene rubber, natural rubber, polyisoprene, poly-(ethylene propylene diene monomer), and the like.
The invention further provides a method for preparing the anionic polymerization initiators described above, comprising the step of reacting (i) an alkali metal compound having the formula R(Me), where R represents a hydrocarbyl group containing from one to about 20 carbon atoms, and Me is a Group IA alkali metal atom, with (ii) a particle as defined above, in the presence of (iii) a polar coordinator, to form the reaction product having the formula P(Me)n, where P represents the particle having covalently bonded alkali metal atoms and n is the same as previously described. The molar ratio of the polar coordinator to the alkali metal compound is about 0.03:1 to about 4:1, preferably about 0.05:1 to about 1:1, and more preferably about 0.06:1 to about 0.5:1.